The present invention relates to a method of electrically synthesizing the sense of taste, and more particularly to a method of electrically generating or synthesizing four fundamental qualities of taste, that is, the saltiness, bitterness, sweetness and sourness.
The present inventor formerly invented a method of electrochemically measuring the quality of taste, which is disclosed in a Japanese Patent Application (Laid-open No. 101,397/1979). In this method, attention is paid to the fact that the gustatory function of human beings is similar to the function of a galvanic cell, and the quality of taste of a substance capable of exciting the gustatory sense is determined by using a galvanic cell which contains a dielectric solution corresponding to saliva and is provided with a pair of electrodes immersed in the dielectric solution. That is, an aqueous solution of the above substance is added to the dielectric solution, and the waveform of the potential difference produced between the electrodes (namely, the so-called gustatory potential curve) is measured to determine the quality of taste of the substance.
As to gustatory potential curves obtained by the above galvanic cell and four fundamental qualities of taste, the following facts have been known.
(1) Of the four qualities of taste, the saltiness and bitterness are the fundamental taste. The sweetness and sourness can be obtained by synthesizing the saltiness and bitterness.
(2) The potential curve corresponding to the saltiness has a waveform such as shown in FIG. 1a. The saltiness is sensed at the rising portion of this curve, and is strongly sensed when the time constant of the curve lies in a range from 1.5931 to 0.7963. The 3 dB upper frequency, as defined hereinafter, corresponding to such a time constant lies in a range from 0.0999 to 0.1998 Hz, and a region where the saltiness is very strongly sensed, exists in the vicinity of 0.1 Hz.
A CR circuit having a series or parallel connection of capacitance C and resistance R gives an impedance Z which is a function of .omega.CR, where .omega.=2.pi.f, f is a frequency of the current applied thereto. The value of Z is the maximum when .omega.=0 and ##EQU1## of the maximum value when .omega.=.omega..sub.h or f=fh. The frequency fh is called as "3 dB upper frequency". That is, the gain of the CR circuit is decreased by 3 dB with respect to a current of the 3 dB upper frequency applied thereto as compared with the gain of the CR circuit with respect to a DC current.
(3) The potential curve corresponding to the bitterness has a waveform such as shown in FIG. 1b. The bitterness is sensed at the falling portion of this curve, and is strongly sensed when the time constant of the curve lies in a range from 0.8337 to 0.4762. The 3 dB upper frequency corresponding to such a time constant lies in a range from 0.1909 to 0.3342 Hz. According to actual measurement, however, a region where the bitterness is very strongly sensed, exists in the vicinity of 0.35 Hz.
(4) The sweetness is a quality of taste which is obtained when the saltiness and bitterness coexists and moreover are mixed in a frequency region near 0.35 Hz (namely, the center of the frequency region where the bitterness is very strongly sensed). The potential curve corresponding to the sweetness has such a waveform as shown in FIG. 1C, and the rising portion of this curve has a relatively gentle slope. The sweetness is strongly sensed when the time constant of the curve lies in a range from 10.2645 to 18.7266. The 3 dB upper frequency corresponding to such a time constant lies in a range from 0.0155 to 0.0087 Hz.
(5) The sourness is a quality of taste which is obtained when the saltiness and bitterness coexist and moreover are mixed in a frequency range near 0.1 Hz (namely, the center of the frequency range where the saltiness is very strongly sensed). The potential curve corresponding to the sourness has such a waveform as shown in FIG. 1d, and the rising portion of this curve has a steep slope. The sourness is strongly sensed when the time constant of the curve lies in a range from 5.1023 to 3.0722, and the 3 dB upper frequency corresponding to such a time constant lies in a range from 0.0317 to 0.0518 Hz.
(6) The boundary frequency between the sweetness and sourness lies in the vicinity of 0.15 Hz.
These facts have been found on the basis of the invention disclosed in the previously-referred Japanese Patent Application, that is, the invention that the four fundamental qualities of taste can be electrically measured by using a galvanic cell. The gustatory sense of human beings is basically caused by an electric stimulus which is generated by a galvanic cell. In view of this fact, the present invention has been made on the basis of the thought that a person may feel the same sense as given by a substance capable of producing a taste, when an electric stimulus is given to the person. If the same gustatory sense as given by the substance having a taste can be generated by the electric stimulus, such generation of gustatory sense will be used in various fields such as the analysis of various kinds of tastes and the diagnosis of gustatory function of human beings. That is, electronics will be utilized in the study of gustatory sense which is behind the studies of visual sense, auditory sense and others, and thus a great industrial effect will be expected.